1. Field of the Invention
The present invention relates to a network system for transmitting audio information, video information, etc.
2. Description of the Related Arts
The case of possessing plural personal computers (the personal computer hereinafter is called “PC”) in one household, and constructing and utilizing a PC network within a home is increasing. Further, a utilizing method for accumulating audio information, video information, etc. in a server device by using the PC network within the home, and reproducing the audio information, the video information, etc. by a client device is considered.
As one example of the utilizing method of the network within the home, there is a method for utilizing Transmission Control Protocol/Internet Protocol (hereinafter called “TCP/IP”) as a communication protocol of the network, and compressing and transmitting the audio information, the video information, etc. in a Moving Picture Experts Group-2 (hereinafter called “MPEG-2”) system. In this utilizing method, there are two types as to which has the leadership of data transmission between the server device and the client device.
One type is called a pull type. In the pull type, the client device has the leadership of the data transmission. When the client device requires data, the client device requests the server device to transmit and output the data. This pull type is used when a compression file, etc. already stored to the server device are read by the client device and are reproduced, etc.
The other type is called a push type. In the push type, the server device has the leadership of the data transmission, and sends data to the client device. This push type is used when the server device compresses and stores the audio information, the video information, etc., and transmits such information to the client device, and the client device reproduces the transmitted audio information, video information, etc.
In the network system of the push type, it is important to take synchronization of the server device and the client device. When a clock used to encode data in an encoder of the server device and a clock used to decode data in a decoder of the client device are not synchronized with each other, an overflow or an underflow is caused in a buffer memory built in the decoder of the client device.
When data transmitted by a communication line of the network are e.g., data of the MPEG-2 system, time information called a Program Clock Reference (hereinafter called “PCR”) is included within the transmitted data. The decoder of the client device takes the synchronization with the encoder of the server device by using the PCR.
However, when the data are transmitted through the communication line, it must cope with a change and a delay of transmission time. There is a case in which other electronic devices are also connected to the network in addition to the server device and the client device and are used. The change and the delay of the transmission time are also caused when such plural electronic devices are connected and utilized.
There are “a clock change compensating method and a real time audio/visual system” disclosed in Japanese patent publication No. 2000-92130 A (corresponding patent EP 0987894 A2) literature 1 as a method for coping with the change and the delay of the transmission time of data. In this method and this system, the storing amount of a buffer memory of the client device is detected and the server device and the client device are synchronized with each other.
The data transmission of a constant bit rate in the network is set to a premise in the invention disclosed in the above literature 1, but this disclosed invention cannot cope with the data transmission of a variable bit rate. This is because the data amount inputted to the buffer memory is changed in the data transmission at the variable bit rate so that no correcting amount can be calculated by detecting only the data amount of the buffer memory.
An object of the invention is to take the synchronization of a server clock signal of the server device and a client clock signal of the client device without having any influence of the change and the delay of the transmission time of data even when the data of audio information or video information, etc. are transmitted at the variable bit rate in the network system of the push type.